Olefins, e.g., propylene or isobutene are used to prepare a number of higher value products such as aldehydes, acids and nitrites. Since the price of the corresponding alkanes, i.e., propane or isobutane is lower than that of the olefins, it would be advantageous to be able to produce the higher value product directly from the alkanes.
Attempts have been made to synthesize novel materials to catalyze the selective oxidation of paraffins. One such catalyst is described in U.S. Pat. No. 5,750,760 where it is disclosed that a multinary composition having the empirical formula: EQU Mo.sub.a V.sub.b Sb.sub.c X.sub.x O.sub.n
where X is an element such as Nb, Ta, W etc. can catalyze the oxidation of an alkane with ammonia in the presence of oxygen. Other compositions which have been disclosed in the art include a Mo.sub.4 VO.sub.14 phase by H. Werner et al. in Catalysis Letters, 44 (1997) 153-63. In J. Catalysis 52, 116-132 (1978), E. M. Thorsteinson et al., describe a mixed oxide catalyst containing molybdenum and vanadium along with another transition metal such as Ti, Nb, Ta, etc. The authors present activity data and physically characterize the compositions. MoVNb systems have also been described in Applied Catalysis, 70 129-148 (1991) and Topics in Catalysis 3, 355-364 (1996). U.S. Pat. No. 4,524,236 discloses a composition containing molybdenum, vanadium, niobium, antimony plus at least one metal such as lithium, barium, titanium etc. U.S. Pat. No. 4,339,355 discloses a composition comprising Mo.sub.a V.sub.b Nb.sub.c X.sub.d, where X is Co, Cr, Cu, Fe, In, Mn and/or Y. It is further disclosed that the compositions have spinel or perovskite structures. In U.S. Pat. No. 4,596,787 a catalyst comprising Mo.sub.a V.sub.b Nb.sub.c Sb.sub.d X.sub.e is disclosed, where X includes Li, Sc, Na, Fr, Ta, etc. U.S. Pat. No. 4,250,346 discloses a catalyst with an empirical formula of Mo.sub.a X.sub.b Y.sub.c, where X is Cr, Mn, Nb, Ta, Ti, V and/or W and Y=Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si, Sn, Ti and/or U. U.S. Pat. No. 4,892,856 discloses a catalyst having the composition Mo.sub.a V.sub.b A.sub.c B.sub.d C.sub.e D.sub.f O.sub.x where A is tungsten or niobium, B is Fe, Cu, Bi, Cr, Sb or Tl, C is an alkali or alkaline earth metal and D is Si, Al or Ti. U.S. Pat. No. 5,807,531 discloses a multimetaloxide having an empirical formula of Mo.sub.12-a-b-c V.sub.a M.sup.1.sub.b M.sup.2.sub.c O.sub.x. However, these materials have a low surface area of 17 m.sup.2 /g or lower. Finally, Ueda et al., in Chem. Commun., 1999, 517-518, disclose a Mo--V--M--O(M=Al, Fe, Cr and Ti) composition which is hydrothermally synthesized. Although these compositions have a diffraction peak at about 3.9 .ANG., they do not have applicant's empirical formula (see below).
In contrast to these reports, applicants have synthesized a new family of crystalline oxide compositions based on niobium, at least one of tungsten and molybdenum, and optionally another metal such as vanadium, tantalum, antimony or tellurium. These novel compositions are prepared hydrothermally and are characterized in that they have an x-ray diffraction pattern with at least one peak at a d spacing of about 3.9 .ANG. and a high surface area. These materials show good activity for converting propane to acrylonitrile.